Sulfur is produced commercially by two techniques: the Frasch process, which involves melting underground deposits of sulfur with superheated water and forcing the molten sulfur to the surface with compressed air, and the Claus process, which takes advantage of the reactivity of sulfur dioxide and hydrogen sulfide to produce elemental sulfur by bringing these two agents together in nearly stoichiometric proportions of two parts hydrogen sulfide to one part sulfur dioxide. Recovery of elemental sulfur from sulfur containing gas streams by the Claus process is a widely practiced procedure wherein elemental sulfur is produced by the well known Claus reaction as follows: EQU 2H.sub.2 S+SO.sub.2 .rarw..fwdarw.2H.sub.2 O+3S REACTION A
Under normal circumstances, the feed gas (acid gas) to the Claus process contains a substantial portion of hydrogen sulfide which is partially oxidized to produce sulfur dioxide in an amount approximately satisfying the stoichiometric relationship indicated by REACTION A. Thus, sufficient oxygen is supplied to the hydrogen sulfide containing stream in a combustion zone to oxidize about 1/3 of the hydrogen sulfide to sulfur dioxide via the following reaction: 2 H.sub. S+30.sub.2 .fwdarw.2 H.sub.2 O+2 SO.sub.2. Alternatively, approximately 1/3 or more of the acid gas stream is diverted to a combustion zone where oxidation of the hydrogen sulfide is carried out in the presence of an amount of oxygen adequate to provide the required amount of sulfur dioxide. The remaining portion of the acid gas stream is not treated so that when the split portions of the gas stream are recombined, the combined gas stream contains hydrogen sulfide and sulfur dioxide in the approximate stoichiometric ratio.
As indicated by REACTION A, three moles of oxygen are required to combust two moles of hydrogen sulfide to water and sulfur dioxide. The sulfur dioxide then reacts with the remaining hydrogen sulfide in one or more reactor(s) or reaction zone(s) over a catalyst such as bauxite or alumina at elevated temperatures to produce elemental sulfur and water vapor. Additionally, during combustion of the hydrogen sulfide, a portion of the hydrogen sulfide in the feed gas dissociates to free hydrogen and elemental sulfur, H.sub.2 S+heat.fwdarw.H.sub.2 +S via thermal decomposition. Any residual hydrocarbons present in the gas stream are oxidized to form carbon monoxide and water vapor which are inert in the Claus reactor.
Oxygen is normally supplied to a Claus unit as pressurized combustion air which typically contains about 21 mole % oxygen and 79 mole % nitrogen on a dry basis. Thus, for every mole of oxygen used to combust hydrogen sulfide approximately 4 moles of nitrogen are introduced in the gas stream. Although nitrogen is inert under typical Claus reaction conditions, the nitrogen contained in the combustion air represents a significant portion of the hydraulic loading of a Claus type sulfur recovery unit.
The amount of oxygen supplied to a Claus type sulfur recovery unit must be controlled to compensate for variability in the volume and composition of the acid gas feed stream(s) to the unit in order to maintain the desired stoichiometric ratio of two moles of hydrogen sulfide per mole of sulfur dioxide in the gas stream entering the reaction zone. The total gas flow through a sulfur recovery unit is, however, limited by the hydraulic capacity of the system. Moreover, from a design standpoint is it not necessarily desirable to design a sulfur recovery unit capable of combusting, with air, the peak volume of acid gas that must be processed due to increased equipment cost and capital expenditure. Additionally, in the case of existing plants, the hydraulic limitations of a sulfur recovery unit may be exceeded if acid gas production is increased due to changes in the process or feedstock. Thus, there exists a need for a method and apparatus that will provide the stoichiometrically required oxygen to a Claus type sulfur recovery unit while simultaneously maintaining gas flows within the hydraulic limits of the unit.
Taggart et al., U.S. Pat. No. 4,919,912, discloses a process for treating sulfur containing gas streams using the Claus reaction in which a recycled stream containing a reactive component is employed in a negative feedback mode to maintain the sulfur producing Claus reaction at approximately equilibrium conditions. The feedstream may contain hydrogen sulfide or sulfur dioxide in a minor amount in an inert gas background. The feedstream to the reaction zone contains a stoichiometrically excess amount of sulfur dioxide for the Claus reaction. Effluent from the reaction zone is passed to a hydrogenation zone where the sulfur dioxide is converted to hydrogen sulfide. Hydrogen sulfide is extracted from the hydrogenation zone effluent and recycled to the Claus reaction zone.
Bond et al., U.S. Pat. No. 3,963,443, discloses a gas mixer and reactor and processes utilizing the mixer and, in particular, a process for converting sulfur containing gas into elemental sulfur.
Bond et al., U.S. Pat. No. 4,051,231, discloses a gas mixer and reactor which includes an elongated gas flow chamber with a nozzle arrangement at its inlet. Atmospheres for kilns having controlled amounts of free hydrogen, carbon monoxide, oxygen, or carbon, for example, are produced by burning controlled ratios of fuel, air, and in some cases an inert gas mixed by the reactor.
Bond et al., U.S. Pat. No. 4,069,020, discloses a process for the production of reducing gases and an apparatus for use therein. A unique gas mixer and reactor is provided which can be used to effect substoichiometric reactions of gaseous reactants to produce a hydrogen-rich gas. The gases which are to be reduced are then admixed with the hydrogen-rich gas, and the gaseous mixture is passed to a catalytic reactor where the reduction reaction takes place.
Bond, et al., U.S. Pat. No. 4,123,220, discloses a gas mixer and reactor which is especially suitable as a burner.
The foregoing references, the disclosures of which are incorporated herein by reference for all purposes, do not, however, address the need for a control system for a Claus type sulfur recovery unit which uses oxygen to control the hydraulic loading of the unit.